(i) Field of the Invention
The present invention relates to the field of the production of hydrogen by the thermocatalytic decomposition (cracking) of hydrocarbons.
(ii) Description of the Related Art
It is known that hydrogen production, in particular on-site, is a major field, especially because of a growing demand in the industrial market for hydrogen, for example for heat-treatment applications.
Many studies have been published in recent years on this subject, particularly studies using catalysts consisting of nickel powder supported on silica (SiO.sub.2).
In particular, the following documents may therefore be referred to:
the article by Zhang et al., published in "Applied Catalysis A," 1998, vol. 167, pp. 161-172; PA1 the article by Muradov, published in "energy and Fuels," 1998, vol. 12, pp. 41-48; PA1 the article by Chen et al., published in the journal "carbon," 1997, vol. 35, pp. 1495-1501; PA1 the article by Poirier et al., published in "International Journal of Hydrogen Energy," 1997, vol. 22. pp. 429-433; or PA1 the article by Steinberg, published in "International Journal of Hydrogen Energy," 1998, vol. 23, pages 419-425. PA1 the problem of carbon deposition after cracking: occurring immediately from the start of cracking, and entailing a significant risk of the reactor becoming blocked (only the first portion of the catalyst is then used); PA1 the difficulties encountered during regeneration of the catalyst: which regeneration is a source of CO and CO.sub.2, therefore of soot deposition; PA1 the regeneration is, moreover, an exothermic process: the increase in temperature and thus the thermocycling resulting therefrom may throw doubt on the integrity of the material; PA1 short regeneration times have to be obtained, especially when it is hoped to achieve truly industrial conditions. PA1 at least one succession, which comprises at least one first and one second catalytic reaction zone, these zones being separate within at least two different consecutive reactors or else being consecutive reactors or else being consecutive within the same reactor, is used; PA1 the at least one first and one second consecutive catalytic reaction zone are subjected to an increasing temperature gradient and/or have an increasing metal concentration gradient in the catalyst; PA1 the initial mixture is made to flow into the first catalytic reaction zone so as to form therein a first intermediate mixture which is directed toward the second catalytic reaction zone of the succession, in order to form the required production mixture. PA1 the succession comprises at least two catalytic reaction zones, each zone being located in a separate reactor; PA1 the succession comprises at least two catalytic reaction zones positioned consecutively in the same reactor; PA1 after a phase in which the production mixture is produced, the process continues with a phase in which the catalytic reaction zones of the succession are regenerated in the following manner: each of the catalytic reaction zones of the succession are independently and simultaneously flushed with the aid of a regeneration gas (for example an oxidizing gas); PA1 after a phase in which the production mixture is produced, the process continues with a phase in which the catalytic reaction zones of the succession are regenerated in the following manner: each of the catalytic reaction zones of the succession are independently and simultaneously flushed with the aid of an oxidizing regeneration gas, the regeneration gas used differing from one zone to another by the fact that it has a different residual oxygen concentration; PA1 after a phase in which the production mixture is produced, the process continues with a regeneration of the catalytic reaction zones of the succession by independently and simultaneously flushing each of the catalytic reaction zones with the aid of a regeneration gas, each of the reaction zones following the first zone of the succession being regenerated in the following manner: a pipe for feeding a regeneration gas is used for each of the catalytic reaction zones which follow the first zone of the succession, each feed pipe being connected to the line used for directing, toward the zone to be regenerated in question, the intermediate mixture produced by the zone preceding it in the succession (the "first zone" and "the following zones" of the succession will be defined by considering the direction of flow of the gas to be cracked in the succession--the regeneration configuration thus described therefore corresponds to concurrent regeneration with respect to the direction of flow of the gas to be cracked in the succession); PA1 after a phase in which the production mixture is produced, the process continues with a regeneration of the catalytic reaction zones of the succession by independently and simultaneously flushing each of the catalytic reaction zones with the aid of a regeneration gas, each of the reaction zones which precede the last zone of the succession being regenerated in the following manner, a pipe for feeding a regeneration gas is used for each of the catalytic reaction zones which precede the last zone of the succession, each of the feed pipes being connected to the line used for extracting, from the zone to be regenerated in question, the intermediate mixture produced by the zone in question (and for directing the intermediate mixture toward the next zone in the succession) (the "last zone" and "the preceding zones" of the succession will be defined by considering the direction of flow of the gas to be cracked in the succession--the regeneration configuration thus described therefore corresponds to a countercurrent regeneration with respect to the direction of flow of the gas to be cracked in the succession); PA1 the phase of regenerating each reaction zone is carried out in the following manner: the flow rate of regeneration gas flushing a given catalytic reaction zone is less than the flow rate of regeneration gas flushing the zone which precedes this given zone in the succession; PA1 the phase of regenerating each reaction zone is carried out in two successive steps of flushing each of thee reaction zones with the aid of the regeneration gas, the regeneration-gas flow rate in the second step being greater than the gas flow rate used for the first step; PA1 before the phase of regenerating each reaction zone, each zone of the succession is purged with the aid of an inert gas; PA1 after the phase of regenerating each reaction zone and before a production phase is started, each zone of the succession is purged with the aid of an inert gas and then each sone of the succession is flushed with the aid of a hydrogen gas; PA1 the regeneration gas coming from one of the zones of the succession is quenched before it flows into the next zone; PA1 the regeneration gas is an oxidizing gas chosen from air, oxygen, CO.sub.2, water vapor and mixtures of these gases; PA1 each of the catalytic reaction zones of the succession is maintained at a temperature lying within the range going from 500 to 1000.degree. C.; PA1 each of the catalytic reaction zones of the succession uses a catalyst based on nickel, cobalt, chromium, iron, platinum, palladium or rhodium; PA1 each of the catalytic reaction zones of the succession uses a catalyst based on supported nickel; PA1 the production gas mixture thus obtained as output from the succession of catalytic reaction zones undergoes one or more purification post-treatments for the purpose of increasing the hydrogen concentration in the mixture, for example by employing a preferential adsorption technique; PA1 at least two successions of catalytic reactions are used, one of the successions being used in production phase while another succession is in regeneration phase, and so on.
The mechanism for hydrogen production under such conditions, which is most commonly accepted in the literature, thus seems to be the adsorption of the hydrocarbon molecule on the surface of a catalyst particle (for example a nickel particle supported on porous silica) followed by the successive dehydrogenation of the hydrocarbon (for example going from CH.sub.4 to CH.sub.3, then CH.sub.2, then CH), in order to end in a carbon atom adsorbed on the surface of the catalyst. This carbon then travels, by thermal diffusion, through the catalyst particle in order to form what are called carbon "nanotubes" or "nanofilaments," a phenomenon allowing the catalyst to be active for a much longer time (the metal surface remains free, accessible and active for a longer time).
It is known that the phenomenon of nanotube formation depends especially on the size of the catalyst particles, on the metal content of the catalyst and on the porosity of the material serving as support for the metal.
This literature can therefore be rapidly summarized by the fact that it has demonstrated the feasibility of the cracking reaction on such catalysts and the activation of the reaction between approximately 550 and 800.degree. C., the fact that maintaining the activity of the catalyst depends essentially on forming these carbon nonofilaments, and that specifically this reaction becomes deactivated when there is so not enough space for these filaments to grow, therefore resulting in the need to regenerate the catalyst, for example by an air flush.
Thus, although all this literature presents the direct catalytic cracking of hydrocarbons as a very promising avenue to explore for the purpose of producing hydrogen (especially on-site), developments undeniably remain to be carried out in order to provide a really industrial process based on this concept, especially when considering the fact that the prior art has obtained all these feasibility results with low gas flow rates and small amounts of catalysts (typically a few milliliters).
Extensive studies by the Applicant have also demonstrated that certain key technical questions still need to be addressed: